Tubulin is the protein that polymerizes into long chains or filaments that form microtubules, hollow fibers that serve as a skeletal system for living cells.
Microtubules have the ability to shift through various formations which is what enables a cell to undergo mitosis or to regulate intracellular transport. The formation-shifting of microtubules is made possible by the flexibility of tubulin which is why scientists have sought to understand the protein's atomic structure since its discovery in the 1950s. Certain anticancer drugs bind to tubulin and cause the protein to lose its flexibility, preventing the cell from dividing.
Regulatory approved tubulin binding agents include the taxanes (including paclitaxel and docetaxel) and the vinca alkaloids (comprised of three agents, vincristine, vinblastine and vinorelbine). Typically these agents are administered intraveneously and are dosed every one to three weeks due to the adverse reactions suffered by patients, including neurotoxicity, neutropenia, hypersensitivity, and other harmful side effects. Thus, there is a continuing need for a dosing regimen that allows tubulin binding agents to be administered for longer periods of time to maximize their anti-cancer effect. Paclitaxel and docetaxel have been shown to have widespread clinical utility in treating tumors; however the clinical decrease in effect over time has limited the usefulness of the drug class. A new taxane-like drug with the capability to overcome the resistance of the tumors may have utility in the clinic.
Clinically used taxanes such as paclitaxel and docetaxel have not been approved for the treatment of brain cancer. The agents do not penetrate the blood brain barrier and remain in the brain sufficiently for them to be effective. This is also the case with many other anti-cancer medicaments so that brain tumors have proved particularly refractory to chemotherapy. The lack of residency in the brain, for example of paclitaxel, may result from efflux related to the P-gp pump.
The design and development of effective anti-tumor agents for treatment of patients with malignant neoplasms of the central nervous system have been influenced by two major factors: 1) the drugs given at high systemic levels are generally cytotoxic; and 2) the blood-brain barrier (BBB) provides an anatomic obstruction, limiting access of drugs to these tumors. Accordingly, it is well known that brain cancers are particularly difficult to treat. The common forms of cancer in the brain are glioblastoma multiforme (GBM) and anaplastic astrocytoma (AA). The mean survival for patients with GBM is approximately 10 to 12 months, while the median survival for patients with AA is 3 to 4 years. For patients with GBM, surgery will prolong their lives only a few months. Most cases where treatment of GBM is by surgery and local irradiation result in relapse within 2 to 4 cm of the original tumor margins.
One current approach to administering a drug that does not cross the BBB into the brain is by craniotomy, a process by which a hole is drilled in the head and the drug administered by either intracerebroventricular (ICV) or intracerebral (IC) injection. With ICV administration, the drug distributes only as far as the ependymal surface of the ipsilateral ventricle and does not penetrate significantly into the brain parenchyma. Therefore, the IVC and IC administration methods reach less than 1% of the brain volume, and there are few diseases of the brain that can be treated by such limited penetration.
In contrast, a transvascular route of drug delivery could treat virtually 100% of the neurons of the brain. Because every neuron is perfused by its own blood vessel, a drug administered tranvascularly can reach every neuron of the brain after crossing the BBB. However, because there is no drug-targeting system that will allow drugs to cross the BBB, the transvascular route of administration is unavailable to the vast majority of drug candidates.
Taxanes are described in the literature including EP 1 228 759; EP 1 285 920; EP 1 148 055; WO 01/56564; WO 01/57027; WO 94/10996; FR 2 715 846; U.S. Pat. No. 5,352,806; FR 2 707 293; WO 94/08984; WO 92/09589; WO 94/20485; WO 93/21173; Klein L L, “Synthesis of 9-Dihydrotaxol: a novel bioactive taxane”, Tetrahedron letters, vol. 34, no. 13, 1993, pages 2047-2050; Datta A et al, “Synthesis of novel C-9 and C-10 modified bioactive taxanes”, Tetrahedron letters, vol. 36, no. 12, 1995, pages 1985-1988; Klein L L et al, Journal of Medicinal Chemistry, no. 38, 1995, pages 1482-1492; J. Demattei et al, “An efficient synthesis of the taxane-derived anticancer agent abt-271”, Journal of Organic Chemistry, vol. 66, no. 10, 2001, pages 3330-3337; Gunda I Georg et al, “The chemistry of the taxane diterpene: stereoselective reductions of taxanes”, Journal of Organic Chemistry, vol. 63, no. 24, 1998, pages 8926-8934. International Patent Application WO 2005/030150 also discloses a series of taxane analogues useful for the treatment of cancer as well as methods of producing them. As used herein, the terms “taxane,” “taxanes,” “taxane derivatives” or “taxane analogs” and the like, include the diterpenes produced by the plants of the genus Taxus (yews), and may be derived from natural sources, may be prepared synthetically or may be obtained from semi-synthetic methods or a combination thereof. Such taxanes include paclitaxel and docetaxel that have a 6-membered A ring, as well as the abeo-taxanes that have a 5-membered A ring, as known in the art and as disclosed herein. The acid catalyzed rearrangement from the 6 membered A ring (and 8 membered B ring) to the 5 membered A ring (and 7 membered B ring) of the abeo taxane has been described for other taxane compounds, for example, in L. O. Zamir et al, Tetrahedron Letters, 40 (1999) 7917-7920, L. O. Zamir et al, Tetrahedron, Vol. 53, No. 47, 15991-16008 (1997), L. O. Zamir et al, Vol. 37, No. 36, 6435-6438 (1996), A. Wahl et al, Tetrahedron, Vol. 48, No. 34, 6965-6974 (1992), G. Appendino et al, J. Chem. Soc., Chem. Commun. 1587-1589 (1993), and G. Samaranayake et al, J. Org. Chem. 1991, 56, 5114-5119. The nomenclature used in these publications to name the rearranged 5 membered A ring structures has been 11(15→1) abeo-taxanes. International Patent Application WO 2005/030150 discloses a series of taxane analogues useful for the treatment of cancer as well as methods of producing them, but no mention is made that these compounds are suitable for use in the treatment of cancers of the brain. Several unique characteristics of both the brain and its particular types of neoplastic cells create daunting challenges for the complete treatment and management of brain tumors. Among these are the physical characteristics of the intracranial space; the relative biological isolation of the brain from the rest of the body; the relatively essential and irreplaceable nature of the organ mass; and the unique nature of brain tumor cells. The intracranial space and physical layout of the brain create significant obstacles to treatment and recovery. The brain is primarily comprised of astrocytes, which make up the majority of the brain mass, and serve as a scaffold and support for the neurons; neurons, which carry the actual electrical impulses of the nervous system; and a minor contingent of other cells, such as insulating oligodendrocytes that produce myelin. These cell types give rise to primary brain tumors, including astrocytomas, neuroblastomas, glioblastomas, oligodendrogliomas and the like.
The brain is encased in the rigid shell of the skull, and is cushioned by the cerebrospinal fluid. Because of the relatively small volume of the skull cavity, minor changes in the volume of tissue in the brain can dramatically increase intracranial pressure, causing damage to the entire organ. Thus, even small tumors can have a profound and adverse affect on the brain's function. The cramped physical location of the cranium also makes surgery and treatment of the brain a difficult and delicate procedure. However, because of the dangers of increased intracranial pressure from the tumor, surgery is often the first strategy of attack in treating brain tumors.